Storage stable, foamable two-part polyorganosiloxane compositions

Abstract

Polyorganosiloxane foams are obtained by blending, dispensing and curing two-part polyorganosiloxane compositions comprising a liquid hydroxyl endblocked polydimethylsiloxane, a liquid alkyl orthosilicate as the curing agent, a catalytically effective amount of a tin compound, a blowing agent and, as a foam stablilizer, a resinous organosiloxane copolymer comprising trimethylsiloxy units, SiO 4/2 units and a specified class of fluorine-containing organosiloxane units.

That which is claimed is: 
     
       1. A two-part polyorganosiloxane composition which when combined to form a homogeneous, flowable mixture yields a cured foam under atmospheric pressure and at a temperature above the boiling point of the blowing agent contained therein, said composition comprising (A) a hydroxyl endblocked polydimethylsiloxane exhibiting a viscosity of from 0.1 to 100 Pa.s at 25° C.;   (B) a liquid alkyl orthosilicate or partial hydrolyzate thereof in an amount sufficient to cure said polydimethylsiloxane with the proviso that ingredients (A) and (B) are maintained in separate parts of said composition;   (C) an amount of a compatible tin compound sufficient to catalyze the reaction of (A) and (B);   (D) a blowing agent maintained under superatmospheric pressure and having a boiling point of no higher than 25° C. under atmospheric pressure, the amount of (D) being sufficient to convert said homogeneous flowable mixture to a foam under atmospheric pressure; and   (E) a foam stabilizer in an amount sufficient to stabilize said foam during the curing process, where (E) consists essentially of a resinous, benzene-soluble organosiloxane copolymer consisting essentially of SiO 4/2  units, silicon-bonded hydroxyl groups, (CH 3 ) 3  SiO 1/2  units and fluorine-containing units selected from the group consisting of R a  R' b  SiO.sub.(4-a-b)/2, R"[Si(R') b  O.sub.(3-b)/2 ] 2  and mixtures thereof, where R is a monovalent organic radical containing at least four perfluorinated carbon atoms and is bonded to the silicon atom of said fluorine-containing units by a sequence of at least two methylene units or by a silicon-bonded oxygen atom that is, in turn, bonded to a sequence of at least two methylene units, R' is an alkyl radical containing from 1 to 3 carbon atoms, R" is an divalent organic radical containing at least four perfluorinated carbon atoms and is bonded to the silicon atoms of said fluorine-containing units by a sequence of at least two methylene units or by a silicon-bonded oxygen atom that is, in turn, bonded to a sequence of at least two methylene units, a is 1 or 2, b is 0, 1 or 2, and the sum of a and b is 3 or less with the proviso that when R and R" represent fluorine-containing units bonded to silicon via an oxygen atom the orgnosiloxane copolymer optionally contains repeating units of the formula GSiO 3/2  where G represents the residue obtained by removal of the hydrogen atom from a hydroxyl group of a linear organic polymer containing an average of at least one terminal hydroxyl group per molecule and selected from the group consisting of polyethers, polyoxyalkylene glycols, homopolymers of ethylenically unsaturated alcohols and copolymers of ethylenically unsaturated alcohols with ethylenically unsaturated hydrocarbons; the molar ratio of all units other than SiO 4/2  and hydroxyl groups to SiO 4/2  units is from 0.7:1 to 1.1:1, inclusive, and the concentration of said fluorine-containing units and any GSiO 3/2  units is sufficient to (a) impart a surface tension of less than 2.2×10 -4  newtons per centimeter at 25° C. to a 10 percent by weight solution of (E) in a trimethylsiloxy endblocked polydimethylsiloxane exhibiting a viscosity of 0.01 Pa.s at 25° C. and (b) require the addition of from 0 to 100 percent by weight of xylene to said 10 percent by weight solution to achieve optical clarity at 25° C.   
     
     
       2. A composition according to claim 1 where the viscosity of the polydimethylsiloxane is from 10 to 20 Pa.s at 25° C. 
     
     
       3. A composition according to claim 1 where said alkyl orthosilicate is ethyl orthosilicate or a partial hydrolysis product thereof. 
     
     
       4. A composition according to claim 3 where the concentration of (B) is equivalent to from 1.0 to 1.7 alkoxy groups in (B) per hydroxyl groups present in (A). 
     
     
       5. A composition according to claim 1 where said tin compound is a stannous salt of a carboxylic acid. 
     
     
       6. A composition according to claim 5 where said tin compound is stannous octoate. 
     
     
       7. A composition according to claim 1 where the fluorine-containing units of (E) are of the formula R a  R' b  SiO.sub.(4-a-b)/2 where R is F(C n  F 2n )(CH 2 ) 2  O c  ; R' is methyl; a is 1; b is 0, 1 or 2; c is 0 or 1 and where in each of said fluorine-containing units n represents an integer that is identical or different with respect to the value of n in other fluorine-containing units of (E), the average value of n being from 4 to 20, inclusive. 
     
     
       8. A composition according to claim 7 where c is 0 and (E) is the reaction product of an organosiloxane copolymer, consisting essentially of (CH 3 ) 3  SiO 1/2  units, SiO 4/2  units and silicon-bonded hydroxyl groups, with a mixture of homologous silanes of the formula F(C n  F 2n  CH 2  CH 2  Si(CH 3 ) b  Cl 3-b  or a hydrolyzate of said mixture, where n varies among said silanes and represents an even integer from 4 to 14, inclusive. 
     
     
       9. A composition according to claim 7 where b is 1 and (E) is the reaction product of an organosiloxane copolymer consisting essentially of (CH 3 ) 3  SiO 1/2  units, SiO 4/2  units and silicon-bonded hydroxyl groups with a mixture of homologous fluorine-containing alcohols of the general formula F(CF 2 ) n  (CH 2 ) 2  OH, where n represents an even integer from 4 to 14, inclusive. 
     
     
       10. A composition according to claim 9 where (E) contains up to 10% of GSiO 3/2  units. 
     
     
       11. A composition according to claim 10 where G represents the residue of a styrene/allyl alcohol copolymer. 
     
     
       12. A composition according to claim 1 where (E) constitutes from 1 to 10% of the weight of said composition exclusive of said blowing agent. 
     
     
       13. A composition according to claim 1 where said blowing agent is selected from the group consisting of aliphatic hydrocarbons containing from 1 to 4 carbon atoms, and aliphatic chlorofluorocarbons containing 1 or 2 carbon atoms. 
     
     
       14. A composition according to claim 13 where said blowing agent is isobutane. 
     
     
       15. A composition according to claim 1 which contains a finely divided filler at a concentration level of up to 50%, based on the weight of said composition. 
     
     
       16. A composition according to claim 15 where said filler is silica. 
     
     
       17. A composition according to claim 16 where said composition includes a treating agent for said silica. 
     
     
       18. A composition according to claim 1 where the viscosity of said composition in the absence of said blowing agent is from 1 to 100 Pa.s at 25° C. 
     
     
       19. A composition according to claim 1 wherein both parts of said composition contain said blowing agent and are maintained under superatmospheric pressure in at least one pressurized container. 
     
     
       20. A composition according to claim 19 where said pressurized container is an aerosol can. 
     
     
       21. A composition according to claim 1 where (B) and (C) are contained in different parts of said composition.